1. Field of the Invention
The present invention relates to an ATM switch, and more particularly to a traffic management function in CLP (cell loss priority) control for an ATM switch.
2. Description of the Related Art
If the number of cells incoming to an ATM switch becomes more than an allowable quantity, if a plurality of cells income at the same time, or if a plurality of cells income in a very short time interval, loads of switch resources which process these cells become large. It is therefore necessary to detect a heavy load of switch resources, i.e., a degraded process state and control to receive high priority cells and discard low priority cells.
In a conventional ATM switch which realizes a cell loss priority control function by setting a plurality of threshold values to buffers, after a queue length of cells in the buffer of the ATM switch exceeds a predetermined buffer threshold value, a newly arrived cell of a low priority class (cell of CLP bit=1) is simply discarded as described, for example, in JP-A-4-369139. Although the cell loss probability changes with the cell traffic characteristics, the above conventional cell loss priority control method for an ATM switch simply discards cells without considering the cell traffic characteristics at all.
With the above conventional cell loss priority control method for an ATM switch, if the traffic characteristics degrade more than the designated value, it is impossible to suppress the cell loss probability of not only a low priority class cell but also a high priority class cell (cell of CLP bit=0) equal to or lower than an allowable cell loss probability dependent on the type of media such as voices and data.
Furthermore, the cell loss probability of the high priority class is not independent from the cell traffic characteristics of the low priority class, but is affected by the cell traffic characteristics of a low priority class. Regardless of the above situations, a switching process is presently performed under the cell loss priority control that cells of communication services guaranteeing the cell loss probability equal to or lower than the allowable cell loss probability are assigned the high priority class whereas cells of best effort services or cells affixed with violation tags by UPC (Usage Parameter Control) are assigned the low priority class. Best effort services are described in "Internetworking with TCP/IP" by D. Comer Prentice Hall, vol. 1, 1991. UPC specifications are set forth in CCITT Recommendation I.311, 1990 (B-ISDN general network aspects).
The above reason for this is that the switching process for the cells of best effort services or cells affixed with violation tags is performed only during the period while the switch resources (buffers and output line capacity) are idle, because the cell loss probability of these cells is not guaranteed.
However, with the above conventional cell loss priority control method for an ATM switch, the cell loss probability of communication services guaranteeing the cell loss probability of the high priority class is adversely affected by the cells of best effort services or cells affixed with violation tags by UPC, and the cell loss probability becomes larger than the allowable cell loss probability (refer to FIG. 26).
UPC is a function aiming to maintain the cell loss probability to a certain level for communication services guaranteeing the cell loss probability. In supporting best effort services, if the cell loss probability for communication services guaranteeing the cell loss probability is degraded, this guarantee is lost. This problem associated with conventional techniques is a fatal disadvantage of ATM switches.
In JP-A-7-58775, the buffer threshold value is changed on the assumption that the cell loss probability of the high priority class is independent of fluctuation in the traffic characteristics of the low priority class. However, in practice, the cell loss probability of the higher priority class is affected by fluctuation in the traffic characteristics of the low priority class. Therefore, this method cannot ensure the cell loss probability of the higher priority class.
In JP-A-6-244858, the buffer threshold value is shifted dependent on the number of arrived cells per unit time. However, for ATM switches, even if the numbers of arrived cells per unit time are the same, the cell loss probability of the high priority class is affected if burstiness and burst lengths are different, so that the cell loss probability of the high priority class cannot be ensured.